Building a bridge: PT or rough cut??

jymbee · May 13, 2020

Finally getting around to putting a bridge together that will span a small stream. Main supports are two railroad railings that will sit on very large, flat rocks on each end. Wondering what would be the difference between 2x PT as opposed to 2in rough cut with respect to overall life span? It will be used as a crossing for our 4WD garden tractor and my thinking is if the width is equal to the wheelbase of the tractor that either would provide sufficient support. Thoughts?

fried1765 · May 13, 2020

jymbee said:
Finally getting around to putting a bridge together that will span a small stream. Main supports are two railroad railings that will sit on very large, flat rocks on each end. Wondering what would be the difference between 2x PT as opposed to 2in rough cut with respect to overall life span? It will be used as a crossing for our 4WD garden tractor and my thinking is if the width is equal to the wheelbase of the tractor that either would provide sufficient support. Thoughts?

What is the span?
What is the tractor weight?
Are you likely to ever get a bigger tractor?

tshep · May 13, 2020

Since you have time to plan,
Use the rough cut and treat it yourself, by soaking, etc.
Gets away from worrying about fastener compatibility, and poor modern consumer 'treatment'.

/pine · May 13, 2020

jymbee said:
Finally getting around to putting a bridge together that will span a small stream. Main supports are two railroad railings that will sit on very large, flat rocks on each end. Wondering what would be the difference between 2x PT as opposed to 2in rough cut with respect to overall life span? It will be used as a crossing for our 4WD garden tractor and my thinking is if the width is equal to the wheelbase of the tractor that either would provide sufficient support. Thoughts?

Rough cut what ??....
...Considering that most PT these days is not worth a darn...and the fact that the majority of it is #2 pine...I would opt for clear white oak (or maybe black locust) if available...it will outlast just about any of the newer PT lumber...

Diggin It · May 13, 2020

Lots of previous bridge threads. Personally, I don't think you want 2x anything unless you plan to laminate several. I think you want 6x something.

Deere Dude · May 13, 2020

jymbee said:
Finally getting around to putting a bridge together that will span a small stream. Main supports are two railroad railings that will sit on very large, flat rocks on each end. Wondering what would be the difference between 2x PT as opposed to 2in rough cut with respect to overall life span? It will be used as a crossing for our 4WD garden tractor and my thinking is if the width is equal to the wheelbase of the tractor that either would provide sufficient support. Thoughts?

I used PT 2x10s for my 18' span bridge but has 8" I beams 24"C-C. Works great and will outlast rough cut wood. At least that is what my research led to.

I would question a span using railroad "railings." Not sure what that is and how long the span is and how everything will be tied together. More info please.

BrokenTrack · May 13, 2020

jymbee said:
Finally getting around to putting a bridge together that will span a small stream. Main supports are two railroad railings that will sit on very large, flat rocks on each end. Wondering what would be the difference between 2x PT as opposed to 2in rough cut with respect to overall life span? It will be used as a crossing for our 4WD garden tractor and my thinking is if the width is equal to the wheelbase of the tractor that either would provide sufficient support. Thoughts?

I built a bridge similar to what you are talking about. I used (2) 5x10's spaced 5'-6" apart for the main supports. This was set a bit wider then my tractor, and a bit narrower than my truck, all made out of cedar and hemlock, rough cut. The planking is made out of 3x6 inches, with boards as runners just to show me where to center the tractor/truck when crossing the short bridge.

With my 3000 pound tractor, or truck, it does not even flex.

Cost was $12.50 as I have my own sawmill. The cost was in buying the spikes which were 6 inches long.

It took me a day to make it, sawing the lumber in the morning, and then framing up the bridge in the afternoon. (I did have the logs hauled out of the woods, and beside the sawmill before the start of the saw/build day.

Sawyer Rob · May 13, 2020

IF the PT you buy is AT LEAST .60 treat, that's what I would use. IF, it's .40, I wouldn't even buy it!

.60 or better PT would be what I would build it out of.

SR

/pine · May 13, 2020

Deere Dude said:
I used PT 2x10s for my 18' span bridge but has 8" I beams 24"C-C. Works great and will outlast rough cut wood. At least that is what my research led to.

I would question a span using railroad "railings." Not sure what that is and how long the span is and how everything will be tied together. More info please.

FWIW...
I have some PT (ground contact) that is less than 4 years old that is rotting...I've seen buried crypts made from hewn white oak that have been in the ground for over 100 years and are as solid as they day they were put there...!